182 THE ELASMOBRANCH FISHES 



to the spiraciilar pseudobranch and that it carries, at least in part, non-oxy- 

 genated blood from the eye; and further that the branch extending from the 

 pseudobranch to the first elf erent-collector is the true efferent pseudobranchial. 

 In the embryo of Squalus acanthias the pseudobranchial (ps., fig. 163) is 

 seen in relation to the remnant of the first embryonic arch, which at this stage 

 has broken, and the segment from the ventral aorta is only a nodule (a.a}). 

 The pseudobranchial itself is somewhat like a cross-trunk in that it is attached 

 to the posterior efferent-collector, but it crosses a relatively long span through 

 the posterior demibraneh of the hyoidean gill and through what would be the 

 anterior demibraneh of the hyoidean gill and the posterior demibraneh of the 

 spiracular gill were such demibranchs present. 



EFFEEENT AETEEIES 



We may now continue the description of the efferent arteries (see figs. 161- 

 162 ) . In the large majority of forms five efferent arteries are present (pentan- 

 ehid sharks). These represent the dorsal parts of the second to the sixth 

 embryonic aortic arches and consist of the hyoidean and four branchial effer- 

 ents. The hyoidean efferent artery passes forw^ard and inward to join the 

 paired dorsal aorta when such persists, or is continued into the head 1)y the 

 orbital (stapedial) artery {Heptanehus, fig. 152). In the embryo of Squalus 

 acamtliias (fig. 163) this vessel is relatively large where it joins the paired 

 dorsal aorta. The four branchial efferents may reach the aorta as four arteries 

 on each side (Selachians) ; or the first maj^ fuse with the second so as to give 

 only three pairs of branches (most rays, fig. 170c). In Hexauchus and in 

 Chlamydoselachus (fig. 160) the fifth branchial efferent joins the fourth 

 before entering the aorta just as, in Heptanehus, the sixth branchial joins 

 the fifth. 



The orbital (stapedial) artery maj- arise at the union of the hyoidean effer- 

 ent and paired dorsal aorta (or., fig. 152) or it may arise near or anterior to 

 this union (fig. 170a). When the latter condition obtains there is formed a 

 common stem from which the orbital and the internal carotid spring. In the 

 adult rays where the paired dorsal aortae may be absent the hyoidean efferent 

 may continue directly into the common stem. 



The orbital (stapedial) artery may reach the orbit without perforating the 

 basis cranii as in Heptanehus, or it may enter bj^ a foramen in the margin of 

 the foramen through which the hyomandibular nerve enters {Heterodontus, 

 }). 59, fig. 66). The orbital gives off one or two branches {Chlamydoselaehus, 

 fig. 160) which supply the muscles of the eye, and a second important branch 

 which passes backward to the hyoid area where it may anastomose with the ex- 

 ternal carotid system. The main stem then passes forward under the eyeball 

 and leaves the orbit through the orbitonasal canal. This stem gives off' a buccal 

 artery which turns downward and backward to supply the adductor mandi- 

 l)ulae muscle and finally the stem divides into the nasal and rostral arteries. 



The internal carotid {i.e., left side, p. 163, fig. 152), which may be consid- 

 ered as the direct continuation of the paired dorsal aorta, enters the cranium 



